The-long term objectives of this research program are to identify those factors that cause or are associated with the decline in sexual activity that accompanies old age. Our research is focused on the decline in sexual behavior and accompanying changes in the hormonal milieu of aging rhesus macaques for what that research may contribute to an understanding of the decline in sexual behavior of aging primates, including man. In a society whose population is steadily growing older, the need for such research grows in importance. Our research is experimental, interdisciplinary, and includes the quantitative assessment of behavioral endpoints following manipulation of environmental, and hormonal variables. Our specific aims are to determine: (1) the extent to which manipulation of environmental conditions such as the social situation and sexual partner can modify sexual performance in old males and hence permit us to assess the degree to which the decline in sexual performance is dependent upon psychological variables (2) the extent to which the decline in sexual performance is related to the competency of the brain- pituitary-gonadal system in old males as that system governs the hormonal environment modulating sexual performance and reproductive function (3) the specific changes in sexual behavior that occur over time with associated changes in serum hormone levels, and if and at what level sexual behavior reaches an asymptote in very old age. Old rhesus males (18-28 years old) whose sexual performance and hormone levels we have monitored since 1970 provide us with a unique opportunity to determine the physiological and psychological correlates of sexual behavior in an aging population of nonhuman primates.